Polypropylene-based multi-layer films are widely used in packaging applications, such as pouches for dry food mixes, pet foods, snack foods, and seeds. Such multi-layer films must have the ability to form reliable hermetic seals at relatively low temperature and, in some instances, the film must do so in the presence of contamination in the seal region, from the contents of the pouches.
Opaque, matte, multi-layer polypropylene film having at least one matte surface which includes at least one base layer and at least one interlayer and an outer layer applied to this interlayer, where the interlayer is a blend of particular olefin polymers and an HDPE or a blend thereof are known.
Multi-layer films providing improved hermetic sealing to packages manufactured in a high speed packaging apparatus wherein the sealant layer provides compliance and adhesivity to the completed seal are also known. In other multi-layer films, improved hermetic compliance and adhesivity functions are provided by separate layers.
Other multi-layer polyolefin films for packaging applications include a heat-sealable surface layer formed of a syndiotactic propylene polymer effective to produce a heat seal with itself at a sealing temperature of less than 230° F. (110° C.) are also known.
Other heat sealable packaging films are laminated structures. The first film comprises at least an ethylene/alpha-olefin copolymer outer first film layer while a second film provides non-heat-shrinkable oxygen barrier. Improved sealing properties have also been achieved in films wherein a soft polymer is blended in both the core layer and a tie layer. Core layers that include a softening additive blended in a core layer to improve compliance of the core layer with the sealable layer in hermetic sealing applications are also known. Some films show improved hermeticity without necessarily, substantially improved minimum seal strength.
Though each of the above films represents a variety of improvements related to packaging films, none of the above films combine desired improvements in processability, seal strength, impact strength, hermeticity, durability, and wide seal ranges for some of today's challenging packaging operations. For example, oriented polypropylene-based films that contain a soft compliant layer create manufacturing problems in the winding operation after orientation and are prone to blocking with the other surface of the film. Such a film will also have a high coefficient of friction (COF), which can cause issues feeding properly through a packaging machine. One known way to resolve the winding, blocking, and COF issues is to use polydialkylsiloxane having a viscosity of 10,000 to 2,000,000 cSt. However, at the levels needed to be effective, there is excessive transfer of the siloxane from the sealant surface onto the opposite surface of the film, typically metal, which results in poor bond strengths in extrusion laminations. Another way to resolve the blocking issues is to use a matte resin in the tie layer (between the core and sealant layer) of the product; however, the COF is still too high, which can cause issues feeding properly through a packaging machine. Another way to resolve blocking issues and reduce the COF is to eliminate the impact copolymer in the tie layer; however, the seal strength of the film will be poor.
None of the above films combine desired improvements in processability, seal strength, durability, hot tack, and wide seal ranges needed for some of today's challenging packaging operations. In particular, a need exists for a multi-layer film that has both good sealing properties and increased surface roughness so as to not be dependent on high loading levels of slip additives. Such a film has the opportunity to replace other packaging substrates, such as paper, foil, and polyethylene laminations in many packaging applications, such as with dry-particulate foods.